1. Field of Invention
The present invention generally relates to a loop filter, and more particularly, to an automatically adjusting gain/bandwidth loop filter that applies to a digital adaptive carrier recovery loop or a digital phase lock loop.
2. Description of Related Art
The phase lock loop (PLL) is used by the quadrature amplitude modulation (abbreviated as QAM) system in conventional communication for carrier recovery operation. However, the phase lock loop with conventional fixed parameters has its disadvantage. When a user intends to increase the carrier tracking speed or the carrier frequency tracking range, a phase lock loop with high gain/bandwidth is required. However, when such method is used for the carrier recovery operation, the jitter generated when entering into the steady-state is also increased. Whereas, the phase lock loop with low gain/bandwidth is required when user intends to lower the steady-state jitter. However, this mechanism delays the time for carrier locking, i.e. delays the pull-in time, and also lowers the maximum residual carrier frequency offset that can be tracked.
A QAM system using a weight computation circuit to modify the loop filter gain is disclosed in xe2x80x9cA New Symbol Synchronizer with Reduced Timing Jitter for QAM systemsxe2x80x9d, Volume: 2, Page 1292-1296, Global Telecommunications Conference IEEE, 1995 by Zhang Hang and Markku Renfors. However, an additional carrier recovery circuit, or a synchronizer, is required by such QAM system for generating a signal for judgment, thus the hardware cost is increased. In addition, the calculated and adjusted gain may be too big, so that the whole circuit is diverged. Moreover, since it only adjusts the loop filter gain and does not adjust the loop filter bandwidth, it cannot provide the best results.
The other carrier recovery circuit is disclosed in xe2x80x9cAdaptive Carrier Tracking Circuitxe2x80x9d of U.S. patent application Ser. No. 4,691,176, where in addition to the complicated channel characteristic estimator, a carrier recovery circuit that causes an additional cost is also used to calculate the situation of the current environment. An output signal from the phase detector is used by the carrier recovery circuit as a source of the judgment signal.
The present invention provides an automatically adjusting gain/bandwidth loop filter. A frequency error in the frequency tracking (integral) branch of the variable gain/bandwidth loop filter is used to estimate a tracking state of the loop without the help of an additional synchronizer. The loop gain/bandwidth is further adjusted based on the tracking state to accelerate the pull-in time and lower the steady-state jitter. Moreover, the circuit can be steadily operated, thus the divergence will not happen anymore.
In order to achieve the object mentioned above and others, the present invention provides an automatically adjusting gain/bandwidth loop filter. The loop filter comprises a variable gain/bandwidth loop filter and a tracking controller. The variable gain/bandwidth loop filter mentioned above filters a noise component included in a first received phase error, and adjusts a gain/bandwidth of the variable gain/bandwidth loop filter according to the tracking control tuning parameter output from the tracking controller, so as to output a second phase error and a frequency error related to a frequency tracking portion in the second phase error. The tracking controller determines a tracking status of the loop that is currently used according to the frequency error, and adjusts and outputs a tracking control tuning parameter used for tuning the gain/bandwidth of the variable gain/bandwidth loop filter.
The tracking controller of the preferred embodiment according to the present invention mentioned above comprises a tracking status detector and a gain/bandwidth control state machine. The tracking status detector determines a tracking status of the loop currently used according to the frequency error, so as to obtain a confidence value that represents the tracking status. The gain/bandwidth control state machine adjusts and outputs a tracking control tuning parameter according to the confidence value. The tracking control tuning parameter comprises a gain tuning parameter and a bandwidth tuning parameter, wherein the gain tuning parameter is used for tuning the gain of the variable gain/bandwidth loop filter, and the bandwidth tuning parameter is used for tuning the bandwidth of the variable gain/bandwidth loop filter.
The tracking status detector of the preferred embodiment comprises a variation calculator, a comparator set, and a confidence counter. The variation calculator calculates a representative variation value that relates to a variation of the frequency error within a time period. The variation calculator comprises a summation unit, a delay unit, an adder, and an absolute value operator. The summation unit receives the frequency error, summates the frequency error within a time period, and outputs a summation value. The delay unit then delays the summation value for a time unit, and the adder subtracts the delayed summation value from the summation value. The absolute value operator then extracts an absolute value from the output value of the adder, so as to obtain a representative variation value. The comparator set compares the representative variation value with the default upper threshold and the default lower threshold, so as to output a counting control parameter. The counting control parameter comprises a downward counting and an upward counting. The comparator set comprises a first comparator and a second comparator. The first comparator compares the representative variation value with the upper threshold, and activates the downward counting if the representative variation value is greater than the upper threshold, so as to decrease the confidence value. The second comparator compares the representative variation value with the lower threshold, and activates the upward counting if the representative variation value is less than the lower threshold, so as to increase the confidence value. As a result, the confidence calculator mentioned above increases or decreases the output confidence value according to the upward or the downward counting of the counting control parameter.
The gain/bandwidth control state machine of the preferred embodiment comprises the following mode steps: an acquisition mode for increasing gain/bandwidth, a hold mode for holding gain/bandwidth, and a steady mode for lower gain/bandwidth. Wherein, when a confidence value is less than a pre-determined default acquisition confidence value, the gain/bandwidth is increased; when the confidence value is greater than the pre-determined default steady-state confidence value, the gain/bandwidth is reduced; otherwise, the gain/bandwidth is held. Furthermore, before the gain/bandwidth of the variable gain/bandwidth loop filter is adjusted to a higher value by a gain/bandwidth control state machine, first determining whether the gain/bandwidth is greater than a pre-determined maximum margin value or not, if it is, the gain/bandwidth is held; or before adjusting the gain/bandwidth of the variable gain/bandwidth loop filter to a lower value, determining whether the gain/bandwidth is less than a pre-determined minimum margin value, if it is, the gain/bandwidth is also held.
The variable gain/bandwidth loop filter of the preferred embodiment according to the present invention comprises a phase tracking branch, a frequency tracking branch, and an adder. The phase tracking branch weight the first phase error and obtains a phase tracking related portion contained in the second phase error according to the tracking control tuning parameter. The phase tracking branch also comprises a multiplier, which is used for multiplying a ratio coefficient of the gain tuning parameter included in the tracking control tuning parameter by the first phase error. The frequency tracking branch operates and processes the first phase error according to the tracking control tuning parameter, so as to obtain a frequency tracking related portion included in the second phase error and obtain a frequency error. The adder then summates the outputs from the phase tracking branch and the frequency tracking branch, so as to obtain a second phase error.
The frequency tracking branch mentioned above comprises a first multiplier, an adder, a delay unit, and a second multiplier. The first multiplier multiplies an integral coefficient of the gain tuning parameter included in the tracking control tuning parameter by the first phase error. The adder then summates the outputs from the first multiplier and the second multiplier, and outputs the result to the delay unit. The delay unit delays the output from the adder to obtain the frequency error. The second multiplier multiplies a bandwidth tuning parameter included in the tracking control tuning parameter by the frequency error, and feedbacks the result to the adder.
In accordance with the other aspect of the present invention, there is provided an automatically adjusting gain/bandwidth method for a variable gain/bandwidth loop filter. The method comprises the steps: first, receiving a first phase error; then, filtering a noise component included in the first phase error mentioned above, so as to obtain a second phase error, and according to a set of the tracking control tuning parameters, adjusting the gain/bandwidth used for the filter operation and obtaining a frequency error that is related to the frequency tracking portion and included in the second phase error; determining a tracking status of the loop that is currently used according to the frequency error, and further adjusting and outputting a tracking control tuning parameter used for tuning the gain/bandwidth mentioned above according to the tracking status.
In accordance with the preferred embodiment of the present invention, the step for determining the tracking status of the carrier recovery loop currently used according to the frequency error further comprises the steps: first, calculating a representative variation value related to the frequency error variation within a time period; then, comparing the representative variation value with the default upper threshold and the default lower threshold, and outputting a counting control parameter; increasing/decreasing and outputting a confidence value that represents the tracking status according to the counting control parameter. The step for calculating the representative variation value further comprises the steps: first, summing the frequency error within a time period, and outputting a summation value; then, delaying the summation value for a time unit; subtracting the delayed summation value from the summation value; extracting an absolute value from the result after subtracting the delayed summation value from the summation value, so as to obtain the representative variation value. In addition, the counting control parameter comprises a downward counting and an upward counting. The step for obtaining the counting control parameter comprises the steps: comparing the representative variation value with the upper threshold, and activating the downward counting if the representative variation value is greater than the upper threshold, so as to decrease the confidence value; comparing the representative variation value with the lower threshold, and activating the upward counting if the representative variation value is less than the lower threshold, so as to increase the confidence value.
In accordance with the preferred embodiment of the present invention, the tracking status comprises an acquisition mode, a hold mode, and a steady mode. The step for adjusting and outputting the tracking control tuning parameter according to the tracking status, so as to further adjust the gain/bandwidth further comprises the steps of: when a confidence value is less than a pre-determined default acquisition confidence value, entering into the acquisition mode so as to increase the gain/bandwidth; when the confidence value is greater than the pre-determined default steady-state confidence value, entering into the steady mode so as to reduce the gain/bandwidth; otherwise, entering into the hold mode so as to hold the gain/bandwidth. Furthermore, in the step mentioned above, before increasing the gain/bandwidth, first determining whether the gain/bandwidth is greater than a pre-determined maximum margin value or not, if it is, entering into the hold mode so as to hold the gain/bandwidth; or before reducing the gain/bandwidth, determining whether the gain/bandwidth is less than a pre-determined minimum margin value, if it is, also entering into the hold mode so as to hold the gain/bandwidth.
Since the present invention utilizes the frequency error to estimate the tracking state of the loop, and further adjusts the loop gain/bandwidth according to the tracking status, thus it can accelerate the pull-in time and lower the steady-state jitter.